Communications systems, especially messaging and paging systems or subsystems, typically operate on a channel having a radio frequency that provides coverage over a generally defined area. The number of subscribers served by these systems is limited by the spectral capacity of the channel where this capacity is the amount of information that the channel may convey. As the number of subscribers grows or the amount of information that a given subscriber or messaging unit wants to convey or receive over the channel increases, there may be no choice other than to deploy additional systems or subsystems having additional channels and frequencies.
These additional systems or channels likely create problems for system operators and messaging units or users thereof. Specifically operating channels for the units and the systems must now be coordinated such that messages may be delivered to a unit without undue waste of system capacity caused by retries or broadcasting the message on multiple channels. Various approaches have been proposed or utilized, such as carrying multiple messaging units each operable on a different channel. Obviously this is overly burdensome and inconvenient. Another approach is providing an ordered listing of channels for the messaging unit at, for example, initial activation. The unit, at power up, searches the ordered list, selects an operating channel, and remains monitoring the highest ordered or highest status channel it can reliably receive. The unit may rescan the list when the current channel is lost or from time to time as in background scanning. This approach is relatively inflexible and usually limited, given practicalities to a two or maybe three channel situation.
The system may duplicate messages on multiple channels but this inefficiently uses channel capacity and basically defeats the purpose of the extra channels. One approach has been proposed by Gaskill, et al. U.S. Pat. No. 4,897,835. Gaskill et al. suggests that the messaging system tells the messaging unit to tune to a particular channel. Subsequent messages are broadcast only on that channel frequency. Drawbacks to Gaskill include sending the tuning message on all channels the messaging unit may be on, thus wasting capacity, and the possibility is that the messaging unit does not receive the tuning message thus missing subsequent messages.
Clearly a need exists to automatically allow a system and a messaging unit to adequately and routinely coordinate, modify, and effect the operating channels available to and used by messaging units.